The present invention is related to circuit boards in which Electromagnetic Interference (hereinafter referred to as xe2x80x9cEMIxe2x80x9d) can be suppressed, and more specifically to a microstrip-type of circuit board which is used as the driver of a liquid crystal display.
A liquid crystal display is equipped with a source driver card and a gate driver card for displaying images. An EMI problem may be caused by the source driver card having a video signal with a maximum fundamental frequency of about 33 MHz and a clock signal having a frequency about two times thereof being provided to a signal input point Pi, for instance, for a display screen of about 1024xc3x97768 pixels.
A digital signal having a frequency of, for example, 33 MHz contains, as harmonic components, high-frequency signal components having frequencies that are integral multiples (n) thereof. The magnitude of each harmonic component is indirectly proportional to n for a signal having a signal duty of, for example, 50%. However, in order to provide a signal quality that is required for a digital signal, it is necessary to have harmonic components of a sufficiently large n (for example, n=5). A signal having only a fundamental frequency, or a harmonic component with an integral multiple of one, is typically a pure sinusoidal wave, and usually does not meet requisite criteria, for example rise time, to be recognized as a digital signal. If the frequency of a signal is relatively low such that there are harmonic components of between about 30 MHz to 1 GHz corresponding to a sufficiently large n such that sufficient signal quality is provided, EMI problems can be avoided by transmitting the signal while suppressing the harmonic components by using a filter or the like. However, given that the fundamental frequency is about 33 MHz means that use of such a filter would suppress both radiated EMI and the fundamental frequency. Therefore, additional means are needed, for example, covering the whole device with an electromagnetic shield. However, use of an electromagnetic shield is not practical as it would shield the front display portion thereby preventing the screen from being seen.
A conventional EMI suppression measure in a source driver card is that of providing the source driver card with a strip line structure, in which both surfaces of the card are grounded using a metal foil, or one surface is grounded and the other surface is electrically connected to a power supply, but this approach can be costly and the resulting card can become thicker than desired.
Another measure is that of utilizing a microstrip structure for a source driver card, that is, one entire circuit layer of a source driver card is grounded and utilized as a ground plane. However, if this approach is taken, there is a problem if there are openings on the ground plane as a return current on the ground plane may detour around the openings causing a common-mode current. Common-mode current is defined hereinbelow. In a loop circuit comprising a signal driver source, a load, a signal line connecting the signal side terminals of the signal driver source and the load, and a current return path connecting the ground terminal of the load and the ground terminal of the signal driver source, the signal driver source provides a high-frequency signal. In this loop circuit, through the effects of surrounding metallic objects, differences between the shapes of the signal line and the current return path, and the like, a current difference is created between current flowing in the signal line and current flowing in a particular portion of the current return path as measured at a particular moment in time. This current difference is known as the common-mode current of that portion at that moment. The electromagnetic wave caused by the common-mode current is very large as compared with the radiation caused by a loop current of the same amplitude, and is a primary cause of EMI.
Publication of Japanese Unexamined Patent Application No. 11-121967 discloses that degrees of symmetry xcex4 of four-terminal circuits (for both an electric circuit and a power circuit containing a transmission line) can be made equal so as to suppress EMI. However, the symmetry disclosed in this publication teaches consideration of symmetry for circuit constants between electric circuits and power circuits containing a transmission line, which is substantially different from a spatial arrangement as taught by the present invention described hereinbelow. Furthermore, the method taught by the publication is that of determining the circuit constant of the transmission line with respect to the circuit constant of the electric circuit. This method does not suppress EMI by determining the structure of a transmission line such as a signal bus or a ground surface.
It is a primary object of the present invention to provide a circuit board that suppresses EMI emanating from the circuit board.
It is another object of the invention to provide a circuit board that suppresses EMI emanating from the circuit board by the circuit board having a substantially symmetrical structure.
It is yet another object of the invention to provide a circuit board that reduces EMI generated by a clock signal as compared to many known such circuit boards.
It is yet another object of the invention to provide a circuit board with a reduced number of layers as compared to many known such circuit boards.
In accordance with one embodiment of the invention, there is provided a circuit board which comprises a ground layer, at least one circuit layer and at least one insulation layer between the ground and circuit layers. The at least one circuit layer has at least one signal bus that extends in a substantially straight line in a longitudinal direction of the circuit board and is adapted for being supplied with a high-frequency signal. The ground layer has openings therein. The at least one signal bus has a signal input point adapted to receive the high-frequency signal which has a frequency higher than a particular value and a plurality of signal output points adapted to transmit the high-frequency signal. The plurality of signal output points are disposed substantially symmetrically with respect to the signal input point in the at least one signal bus and the openings are disposed substantially symmetrically with respect to the signal input point to provide EMI suppression for the circuit board.
In accordance with another embodiment of the invention, there is provided a circuit board which comprises a ground layer, at least one circuit layer and at least one insulation layer between the ground and circuit layers. The at least one circuit layer has at least one signal bus that extends in a substantially straight line in a longitudinal direction of the circuit board and is adapted for being supplied with a high-frequency signal. The ground layer has openings therein. The at least one signal bus has a signal input point adapted to receive the high-frequency signal which has a frequency higher than a particular value and a plurality of signal output points adapted to transmit the high-frequency signal. The plurality of signal output points are disposed substantially symmetrically with respect to the signal input point in the at least one signal bus and the openings are disposed substantially symmetrically with respect to the signal input point to provide EMI suppression for the circuit board. The circuit board further includes a side edge which has a plurality of tab modules located thereon which have pins thereon. The tab modules extend in a longitudinal direction of the circuit board and the plurality of signal output points of the at least one signal bus are correspondingly connected to the pins on the plurality of tab modules through a plurality of tab leads. The plurality of tab leads are disposed substantially symmetrically with respect to a straight line passing through the signal input point of the at least one signal bus and substantially perpendicular to the at least one signal bus.